Resin materials have become widespread in all kinds of products from industrial products to domestic articles, various kinds of processing are applied to each resin material to provide its own function, and a high processing accuracy is demanded. However, as pore processing with respect to a resin, there are drilling, press working, electron beam machining, etching processing and others, but forming a through hole having a hole diameter that is not greater than a material thickness and forming a through hole at a hole pitch that is not greater than a material thickness are difficult.
An application field of a laser processing application technology has spread to cutting/boring/welding using a CO2 laser, precise cutting/boring/welding using a YAG laser, marking, removal of a thin film by using an excimer laser and high-polymer material processing, and this technology is also applied to the above-described resin material processing field.
Here, when a hole is formed in an article made of a resin material by using a CO2 laser which is an infrared light, there occurs a problem that processing with a fine hole quality cannot be performed because of a raise formed around the through hole due to a thermal influence or attachment of an ejection or that holes adjacent to each other are connected if a hole pitch is too narrow, thus disabling hole processing with a high density.
At the present day, practical application of a machining process using an UV light source such as a third harmonic/fourth harmonic of a YAG laser has advanced, and development utilizing an ablation phenomenon that chemical bond in constituent molecules of a high-polymer material is cut based on photon energy by using a laser having a wavelength in a UV region and a portion irradiated with a laser beam is instantaneously decomposed and scattered is in progress.
Processing utilizing this ablation phenomenon has many merits, e.g., enabling precision processing with less thermal influence because of a small thermal diffusion amount with respect to a peripheral portion, eliminating the need for post-processing or finish processing and others, and hence use of this processing is rapidly spreading.
However, in the hole forming processing of forming a through hole in a processing target (work object), there has been reported that a diameter of a formed through hole on a laser beam exit side of the processing target is smaller than that on a laser beam incidence side, i.e., the hole is formed in a so-called tapered shape because of characteristics of the laser beam.
Therefore, formation of a through hole having a larger diameter or formation of more through holes is required to obtain a sufficient opening area and, at this time, there occurs a problem, e.g., a reduction in mechanical strength of a processed portion, a reduction in processing efficiency, an increase in processing cost, and eliminating securement of a hole area rate required in a specified area in design.
There has been demanded a new through hole forming method that enables controlling a shape of a through hole to be formed, e.g., enabling formation of a through hole whose diameter remains the same on a laser beam incidence side of a processing target and on a laser beam exit side of the same or enabling formation of a through hole having a so-called inverse tapered shape that a diameter of the formed through hole is larger on a laser beam exit side of the processing target than on a laser beam incidence side of the same.
Here, Japanese Patent Application Laid-open No. 2003-158358 (Patent Document 1) discloses a method of contact-bonding and attaching a protective sheet to a back surface side of a processing target to provide a double structure, irradiating the processing target with a laser beam, forming a hole in the protective sheet partway and then delaminating the protective sheet to obtain a through hole.
Further, Japanese Patent Application Laid-open No. 2004-243404 (Patent Document 2) discloses a technology that vertically irradiates a substrate with a laser beam to form a tapered hole, then irradiates the substrate with a laser beam at a slat and uniforms a hole diameter on an incidence side and that on an exit side to form a straight hole.
Furthermore, Japanese Patent Application Laid-open No. 58051-1999 (Patent Document 3) discloses a technology that sets a focal position of a laser beam above an insulative substrate in a state that a reflecting material (a copper foil) is arranged on a back surface side of the insulative substrate and applies a laser beam, whereby a hole is gradually formed from an upper surface of the substrate and a tapered hole reaching the copper foil is eventually formed. When the irradiation is further continued, the laser beam is reflected on the copper foil to strike on an inner surface of the tapered hole, thereby increasing a diameter of the hole and finally straightening the through hole.
Moreover, Japanese Patent Application Laid-open No. 2001-79677 (Patent Document 4) discloses a technology that bends and laser-processes a synthetic resin elastic sheet at a laser beam irradiating portion to form a hole having a diameter increased on a laser beam exit side and flattens this sheet after the processing, thereby straightening the through hole.
Additionally, Japanese Patent Application Laid-open No. 141766-1996 (Patent Document 5) discloses a technology by which a high-reflectivity plate is arranged below a processing target, a through hole is formed in the processing target by an applied laser beam, and then the incident laser beam is reflected by the high-reflectivity plate, this reflected laser beam strikes on a through hole inner surface or a notch inner surface to effect laser processing, thereby obtaining the through hole or the notch having no tapered shape.
However, according to the technology disclosed in Patent Document 1, hole diameters on the laser beam incidence side and the laser beam exit side are determined based on a thickness of the processing target, and a difference between the hole diameters increases as the thickness rises, whereby forming the through hole having a so-called straight shape becomes difficult. Further, a freedom degree of a shape of the processing target is lowered. Furthermore, attaching the protective sheet to the processing target means that a step of attaching the protective sheet and a step of delaminating the protective sheet after the laser processing are added. Moreover, there are problems of an increase in cost due to the protective sheet and a waste material since the delaminated protective sheet cannot be recycled. Additionally, according to the technology disclosed in Patent Document 1, there is no description or suggestion about a method for forming a through hole having an inverse tapered shape, and forming such a hole is practically impossible by this technology.
Further, according to the technology disclosed in Patent Document 2, the apparatus and its control are complicated and sophisticated since the substrate is vertically irradiated with the laser beam and then the irradiation is performed while inclining and rotating the laser beam emitting unit or the processing target, and hence there occurs a problem that a product cost increases.
The method disclosed in Patent Document 3 has a concern that a processing accuracy is lowered due to a reduction in reflectivity of the reflecting material when the reflecting material is repeatedly used since the reflecting material arranged on the back surface side of the insulative substrate is exposed to the laser beam every time. Furthermore, when applying this method to a thin material in which the processing target is formed of a resin, an effect of the method cannot be expected unless the processing target and the reflecting material are sufficiently in close contact with each other, a freedom degree of a shape of the processing target is lowered, and steps of the contact bonding and the delamination of the reflecting material are added, thus resulting in an increase in cost.
In the method disclosed in Patent Document 4, since the processing target must be bent in accordance with a tapered shape obtained in the laser irradiating portion and a curvature must increase as a thickness of the processing target rises, the processing target may possibly crack or bend at the time of processing, and there is a drawback that this method cannot be practically applied to articles other than those having a tabular shape like a sheet, e.g., cases or various kinds of housings.
In the method disclosed in Patent Document 5, the high-reflectivity plate is arranged below the processing target, but processing using the reflected laser beam may become uneven unless the processing target and the high-reflectivity plate are in close contact with each other. Further, since the reflecting plate is exposed to the laser beam, the processing accuracy may be lowered as the reflectivity is reduced.
As described above, there is not a technology that can be called a through hole forming method which can easily control a shape of a through hole to be formed, e.g., formation of a through hole having a through hole diameter which remains the same on a laser beam incidence side and a laser beam exit side of a processing target irrespective of a shape of the processing target or formation of a through hole having a so-called inverse tapered shape that a diameter of a formed through hole is larger on a laser beam exit side of the processing target than on a laser beam incidence side depending on a use application.
Furthermore, in the conventional technology, as described above, although a minimum value of a distance between axes of through holes (between central axes of through holes) is determined based on a size of an incidence-side diameter of each through hole when adjacently providing the plurality of (many) through holes since a shape of the through holes is smaller on the laser beam exist side than on the laser beam incidence side, the distance between the axes of the through holes cannot be reduced because the incidence-side diameter of each through holes is large. When such a conventional technology is applied to a device in which a size of each through hole must be set to an invisible size, e.g., 100 μm or below in design such as a light transmission unit (a display apparatus and others) or a sound transmission unit (a speaker, a microphone and others) in a mobile phone or a mobile music device, there is a problem that a hole area rate cannot be increased and sufficient light or sound cannot be transmitted.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-158358    Patent Document 2: Japanese Patent Application Laid-open No. 2004-243404    Patent Document 3: Japanese Patent Application Laid-open No. 58051-1999    Patent Document 4: Japanese Patent Application Laid-open No. 2001-79677    Patent Document 5: Japanese Patent Application Laid-open No. 141766-1996